Organic Synthesis for Sustainable Biofuels


"Biofuels play an essential role in reducing carbon emissions from transportation" says Adrian Higson, Energy Consultant. But there are still problems with current biofuels. They have a lower energy densitymade from potential food crops, such as wheat and sugar cane.

So researchers are aiming to develop more sustainable and efficient biofuels, known as advanced biofuels. One approach is to produce biofuels from non-edible, lignocellulosic (woody) biomass, such as agricultural wastes or forestry residues. Another is to develop chemical reactions to remove oxygen from biofuels to create biofuels which can be blended with existing fuels, called ‘drop-in biofuels’.

Platform molecules

Developing renewable fuel sources is challenging, and organic synthesis is a vital tool in this process. Chemists aim to isolate ‘platform molecules’, which can be produced from inedible biomass and then converted into biofuels. They then design chemical processes to alter the structure and function of these molecules to convert them into efficient fuels.

Two important examples of platform molecules are levulinic acid and furfural (Figure 1). Levulinic acid is produced from bio-polymers such as cellulose, in yields of over 70%. Cellulose is inedible, but is the most common organic compound on Earth3, as it is a crucial component in green plant cell walls. Furfural is produced from agricultural wastes like corn cobs, which contain hemicelluloses.

While currently bioethanol is the most common biofuel, levulinic acid and furfural are more often used to produce bio-gasoline or biodiesel, which have higher energy densities. These fuels have great potential – production of biodiesel is expected to increase from 11 billion litres to 24 billion litres by 2017.

Valuable conversions

Levulinic acid can easily be converted by hydrogenation into γ-valerolactone, the 5-membered ring shown below (Fig. 2). γ-valerolactone can be converted into a wide range of useful biofuels, and polymers. For example, straight chain hydrocarbons can be made through Dumesic’s approach. This method reduces the oxygen content of the molecule as well as increasing the molecular weight, creating a more efficient fuel. These straight chain alkanes can be added to diesel and petrol fuels4.

Alternatively, hydrogenation followed by esterification can lead to valeric esters, which have a relatively high energy density and can be used as biodiesel fuel additives (Fig. 3).

There are some ingenious synthesis methods used to create biofuel molecules. One of these is Corma’s synthesis, which generates a long, C-15 hydrocarbon from furfural. This branched C-15 hydrocarbon is a high quality, sustainable biodiesel molecule (Fig. 4)

For organic chemists, there are significant opportunities associated with further developing energy crops and producing advanced biofuels from new sources such as algae, industrial or post-consumer waste.

References

1 J Q Bond, D M Alonso, D Wang, R M West and J A Dumesic, Science, 2010, 327, 1110
2 A Corma, O de la Torre, M Renz and N Villandier, Angew. Chem. Int. Ed., 2011, 50, 2375
D Klemm et al., ChemInform, 2005, 36
J C Serrano Ruiz and J A Dumesic in Catalysis for Alternative Energy Generation, 2012, 49-50
D M Alonso, S G Wettstein and J A Dumesic, Green Chemistry, 2013, 15, 584


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Contact and Further Information

Dr Anne Horan
Programme Manager, Life Sciences
Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge, CB4 0WF
Tel: 01223 432699